Bias instability in Heathkit W-5M amp - diyAudio
Go Back   Home > Forums > Amplifiers > Tubes / Valves

Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 26th September 2007, 07:55 PM   #1
ltarini is offline ltarini  United States
diyAudio Member
 
Join Date: Sep 2007
Default Bias instability in Heathkit W-5M amp

Hello, I have a Heathkit W-5M that will not set bias. The W-5M does not have a bias control to set overall current, only a balance control to match the current in the two KT66 (China) output tubes. However, as the amp warms up no amount of adjustment will keep the balance at 0, clearly one tube is drawing too much current, so I shut the amp off immediately.

The amp has been modified by a prior owner, so I need to check if any cathode resistors or grid resistors have changed value in a way that might create excess current. However, there is some evidence of tar leakage from the Peerless 16458 output transormer, so I wonder if there might be something amiss inside there. I don't know a good way to check that without removing the transformer.

I have heard that too high a grid resistor value can cause grid current (electrons flowing from grid to plate) which makes the bias on the grid less negative, causing current to increase, and I have also heard that some Chinese tubes are particularly susceptible to this. Should I try lowering the grid resistor values? Or, alternately, try raising the cathode resistor values? Most of all, the leaked tar from the output worries me and I would like to know how I can test the transformer, even if it involves removing it from the amp.

Thanks for your suggestions.

Schematic for the Heathkit W-5M can be found here:

http://www.vintage-radio.info/heathkit/index.htm
  Reply With Quote
Old 27th September 2007, 03:44 PM   #2
diyAudio Member
 
kstagger's Avatar
 
Join Date: Dec 2003
Location: Grand Rapids
I've heard a pair of Heath W5s a few years ago that was converted to fixed bias. They added a small transformer, diodes and capacitors to provide the negative voltage.

If you have another pair of output tubes, you may want to try those - you could have some seriously mismatched output tubes and/or some improper modifications.

and Hello fellow michigander!
  Reply With Quote
Old 27th September 2007, 04:02 PM   #3
diyAudio Member
 
Tom Bavis's Avatar
 
Join Date: Jun 2002
Location: Macedon NY
Check the output transformer with an ohmmeter - if the plate section opens, the tube will still draw current thought the screen, which will overheat...

It's also possible that there's a high-frequency oscillation... hard to tell without a 'scope. Grid resistors in this are pretty low - I wouldn't expect any problem there. But as mentioned, tubes could be a problem.

I have the assembly manual on my page: http://www.audiophool.cjb.net/hifistuff.html
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Naim NAP250 Regulator Instability Problem ceharden Solid State 9 29th April 2008 10:22 PM
EL-34 instability in EICO HF-35 amp ltarini Tubes / Valves 0 26th September 2007 06:43 PM
Help! Phono stage LF instability Jeb-D. Tubes / Valves 2 25th August 2007 10:39 PM
Output stage instability Katapum Solid State 35 27th December 2005 05:31 PM
Zen instability? peted Pass Labs 10 27th August 2001 10:21 AM


New To Site? Need Help?

All times are GMT. The time now is 03:04 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2